1. Field of the Invention
This invention relates to electronic AC electric energy measuring circuits and more particularly to such circuits including improved circuits for multiplying the voltage and current components of an alternating current energy quantity to be measured by uniquely utilizing one of two analog input signals as a time derivative of the current component for producing output pulses representative of quantized amounts of electric energy.
2. Description of the Prior Art
Devices for AC electric energy measurement are extensively used by producers of electric energy for measuring consumption by separate energy users. Typically, watt-hour meters are used for indicating consumption in kilowatt-hours. The watthour meters are usually of the induction type having a rotating disc, which are recognized as having high degrees of reliability and accuracy, being available at reasonable costs, and being capable of outdoor operation under widely varying extremes of temperature and other ambient conditions.
It is also known to measure AC electric energy quantities such as kilowatt-hours, volt-ampere hours, reactive volt-ampere hours, with electronic measuring circuits. Typically, voltage and current sensing transducers provide signals proportional to the voltage and current components of an electric energy quantity to be measured. Analog multiplier circuit arrangements are known in one type of measuring circuit and they are arranged to produce a signal proportional to the time integral of the product of the voltage and current components. One electronic measuring circuit is described in U.S. Pat. No. 3,764,908, assigned to the assignee of this invention, wherein voltage and current signals are applied to a semiconductor device having a logarithmic computing characteristic. An output signal is produced therefrom which is proportional to the product of the voltage and current signals and a measured value of the electric power quantity.
Another known analog multiplier type of electric energy measuring circuit is referred to as a time division multiplication type of measuring circuit. In U.S. Pat. No. 3,864,631, assigned to the assignee of this invention, a further technique of analog multiplication is disclosed. A voltage component signal is sampled to derive a variable pulse width modulated signal corresponding to the voltage component variations. A current component signal is sampled at a rate responsive to the variable pulse width signals. A resultant output is produced, consisting of a series of pulses having amplitudes proportional to the instantaneous current values and pulse widths proportional to the instantaneous voltage values. The resultant signal is filtered to obtain an average value, or DC level, of the pulses which in turn is proportional to an average electric power quantity. The average value signal controls a voltage-to-frequency conversion circuit, utilizing integrating capacitors. Variable frequency pulses from the conversion circuit are totalized, so that a total pulse count is a measure of the electric energy consumption. One of the recognized considerations and limitations of analog multiplying circuits including the time division multiplication type is the effect of changes in the characteristics of capacitor devices required in such circuits. Capacitors having highly linear and constant characteristics under varying temperature and environment conditions and over extended time periods are often quite expensive and difficult to obtain.
In U.S. Pat. Application Ser. No. 919,874, filed June 26, 1978, and assigned to the assignee of this invention, an electric energy measuring circuit and method is described wherein the voltage component of an electric energy quantity to be measured is converted by electric circuit techniques to a signal proportional to the time integral of the voltage component. The time integral voltage signal is compared to incremental reference levels. Each instance that a referenced level is reached, the instantaneous magnitude of the current component is sampled and converted to digital signals. These digital signals are summed to produce an output signal corresponding to a measure of electric energy is watthours. Some of the component drift disadvantages of prior analog multiplier circuits are avoided by the aforementioned circuit.
A further example of an electric energy measuring circuit is disclosed in U.S. Pat. No. 4,077,061, assigned to the assignee of this invention, where analog-to-digital sampling of the voltage and current components is performed for subsequent digital processing and calculation. A number of different electric energy parameters are calculated by digital computational circuit techniques.
In each of the aforementioned circuit techniques for electric energy measurements, the voltage and current inputs to the AC energy measuring circuit are provided directly by the line voltage and current or by potential and current transformers for producing signals proportional to the line voltage and current components of the electric energy quantity being measured. Although electronic circuits are operable in small signal ranges, the electric power voltage and currents are several magnitudes larger. Thus, the sensing transducers to provide the voltage and current responsive analog inputs to the measuring circuits must have large transformation ratios which are linear, and in the case of current sensing transducer, the outputs must be linear over a wide range of current values to be sensed.
In typical local energy usage measurements, sixty hertz AC electric power is delivered at substantially constant line voltages of either one hundred-twenty or two hundred-forty volts defining the voltage component of the electric energy quantity to be measured. On the other hand, load currents which define the load component of the electric energy quantity to be measured vary considerably, typically in a range from one-half ampere to two hundred amperes, or in a variation ratio of approximately four hundred to one. Accordingly, standard potential transformer arrangements generally provide adequate voltage sensing transducers while current transformers operating with the aforementioned input line variations and corresponding low level signal outputs require devices which are of substantial size and cost. When it is desired to manufacture electronic AC energy measuring circuits and devices which are relatively compact and comparable in cost to the aforementioned conventional induction type watthour meters, the voltage and current sensing transducers present substantial contributions to the overall size and cost of such devices. As is known, accurate current transformer transducers require that the ampere turns of the primary and of the secondary must be equal, and since maximum current levels in the primary reach 200 amperes while producing linear low level output analog signals, such current transformer devices are generally bulky and are relatively costly.
In U.S. Pat. Application Ser. No. 923,619, filed concurrently with this invention, and assigned to the assignee of this invention, a mutual inductance current sensing transducer is described and claimed which overcomes some of the aforementioned limitation of current transformers. The current sensing transducer provides an analog signal proportional to the time derivative of the line current so that an associated AC electric energy measuring circuit must be capable of utilizing the time derivative current signal. The present invention is particularly directed to the aforementioned considerations and limitations caused by the variations and drift tendencies of circuit components required in electronic analog multiplying circuits used for measuring alternating current electric energy quantities and to the use of an analog input signal which is proportional to the derivative of line current with respect to time in such analog multiplying circuits for electric energy measurement.